1. Field of the Invention
The present invention relates to the treatment of aqueous solutions of heteropolysaccharides, in particular xanthan gum, to improve the filterability and injectability thereof. This invention especially relates to the treatment of entire fermentation worts destined for the formulation of aqueous fluids adapted to displace petroleum in partially depleted deposits thereof and hence to assist in its recovery.
2. Description of the Prior Art
Heteropolysaccharides or biopolymers obtained by the fermentation of a carbohydrate under the action of bacteria of the genus Xanthomonas or Arthrobacter, or of fungi of the genus Sclerotium, are widely industrially useful because of such properties as capacity to markedly increase viscosity and to serve as thickening agents.
One of the known uses for heteropolysaccharides of xanthan gum type is in the secondary or tertiary recovery of petroleum. In this application, dilute aqueous solutions having a concentration of about 300 to 3000 ppm are used to displace the oil in partially depleted reservoirs thereof. Xanthan gum is characterized by high viscosity at low concentrations, great insensitivity to salinity and to the nature of the salts, and high stability under mechanical stress. However, solutions prepared from industrial grades, either from fermentation worts or by the dilution of a powder which has been precipitated and separated from such worts, have the major disadvantage of rapidly clogging the pores of the rock formations into which they are injected, thereby effecting undesirable rises in pressure and rapidly preventing any additional recovery of oil. It too is known that this clogging is due to the presence, on the one hand, of insoluble particles such as cellular debris and nonviable bacteria originating in the fermentation, and on the other, of a certain number of translucent aggregates or microgels, especially if the solution is prepared from biopolymers which have been precipitated from fermentation worts.
A variety of techniques have been proposed to improve the viscosity and/or the filterability and injectability of xanthan gums, including heat treatments, flocculation, enzyme treatments, etc., whether or not combined with filtration, for example, over diatomaceous earth.
In the process described in U.S. Pat. No. 3,355,447 the liquor is treated at pH 7-9, at a temperature of 65.degree.-77.degree. C. for at least 20 minutes, then diluted and filtered to obtain a clarified hydrophilic colloid solution.
U.S. Pat. No. 3,591,578 describes a wort treatment which involves heating the broth at a temperature of 80.degree.-130.degree. C. for from 10 to 120 minutes at a pH of 6.3-6.9 to increase viscosity.
U.S. Pat. No. 3,729,460 features a treatment of the solution at an alkaline pH, providing a modified structure.
In French Patent No. 2,330,697 a treatment is described: which is carried out at a temperature greater than 100.degree. C. for from 1 to 300 minutes, the salt concentration is at least equal to 0.5% by weight and the solution is filtered. The polymer treated in a saline medium is set forth as being physically different from a polymer which has not been subjected to such a treatment.
The process described in U.S. Pat. No. 3,773,752 entails diluting the fermentation wort, addition of a coagulant and subsequent filtration, and U.S. Pat. No. 3,801,502 entails addition of an alcohol, a phenol or a nonionic surfactant over the course of the heat treatment.
According to French Patent No. 2,440,992 the salt content is less than 0.2% and the heating is carried out at 60.degree.-98.degree. C. for from 2 to 60 minutes.
In each of the processes immediately above described, the heat treatment is carried out either at the natural pH of the fermentation wort or at an alkaline pH.
Cf. published U.K. Patent Application GB No. 2,111,520 A, which relates to improving the clarity and filterability of xanthan gum solution by treating a crude wort at a pH of 2 to 7 with an effective amount of an acid or neutral protease and then raising the pH to 8 to 13, desirably at a temperature of from 50.degree.-70.degree. C. for from 0.5 to 10 hours. In Table 1, comparative Example 1(c), an inoperative experiment is described which is carried out in the absence of enzyme at a temperature of 60.degree. C., at a pH of 5.5, for 1 hour.
In published European Patent Application No. 0,069,523 is described a process for concentrating a xanthan gum type biopolymer solution by ultrafiltration. A crude wort or solution reconstituted from powder is employed, optionally purified (albeit no details of any purification technique are set forth).
U.K. Patent Specification No. 1,488,645 (published French Patent Application No. 2,318,926) is oriented as regards increasing the viscosity of a xanthan gum wort via heat treatment at 99.degree.-104.degree. C., for a critical time period of from 1 to 5 minutes at a pH of 6-7. No indication of filterability characteristics is given, but, as will later be seen, a heat treatment of insufficient duration is ineffective to enhance filterability.
U.S. Pat. No. 4,299,825 relates to clarifying and concentrating a raw Xanthomonas heteropolysaccharide fermentation broth by filtration and ultrafiltration. The wort may be subjected, prior to filtration, to a heat treatment at 60.degree.-150.degree. C. for from 0.3 to 3 hours. The pH of the heat treatment is unspecified, thus being carried out at a natural pH of from 6.5 to 7.5. The clarified solution has a pH of 6 to 9, however, adjusted by addition of an acid or base.
Enzyme action has also been proposed to improve the injectability and filterability of aqueous solutions of xanthan gum.
U.S. Pat. Nos. 3,966,618, 4,119,491 and 4,165,257 describe treatments employing a protease type enzyme. These treatments do not permit completely overcoming the problems of clogging associated with the presence of insoluble proteinaceous substances.
And in European Patent No. 0,039,962 is described a treatment using a complex enzyme having B 1,3-glucanase and protease activity, derived from Pellicularia filamentosa and/or Pellicularia sasakii. Finally, published French Application Nos. 2,491,494 and 2,506,328 proposes an enzyme treatment utilizing polysaccharides and polysaccharase in combination with protease. It has been found that treatment solely with polysaccharase has but a limited effect. The mixed polysaccharase/protease treatment is complicated to use, as the two types of enzyme develop their activity under different pH conditions and at different temperatures.